This invention relates generally to an electrophotographic printing machine and, more particularly, to a development system for development of electrostatic images.
An electrophotographic printing machine includes a photoconductive member which is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to an optical light pattern representing a document being produced. This records an electrostatic image on the photoconductive member corresponding to the informational areas contained within the document. After the electrostatic image is formed on the photoconductive member, the image is developed by bringing a developer material into effective contact therewith. Typically, the developer material comprises toner particles bearing electrostatic charges chosen to cause them to move toward and adhere to the desired portions of the electrostatic image. The resulting physical image is subsequently transferred to a copy sheet. Finally, the copy sheet is heated or otherwise processed to permanently affix the powder image thereto in the desired image-wise configuration.
Development may be interactive or non-interactive depending on whether toner already on the image may or may not be disturbed or removed by subsequent development procedures. Sometimes the terms scavenging and non-scavenging are used interchangeably with the terms interactive and non-interactive. Non-interactive development is most useful in color systems when a given color toner must be deposited on an electrostatic image without disturbing previously applied toner deposits of a different color, or cross-contaminating the color toner supplies.
In the prior art, both interactive and non-interactive development have been accomplished with magnetic brushes. In typical interactive embodiments, the magnetic brush is in the form of a rigid cylindrical sleeve which rotates around a fixed assembly of permanent magnets. In this type of development system, the cylindrical sleeve is usually made of an electrically conductive, non-ferrous material such as aluminum or stainless steel, with its outer surface textured to improve developer adhesion. The rotation of the sleeve transports magnetically adhered developer through a development zone where there is direct contact between the magnetic brush and the imaged surface, and toner is stripped from the passing magnetic brush filaments by the electrostatic fields of the image.
U.S. Pat. No. 5,409,791 to Kaukeinen et al., the disclosure of which is hereby incorporated by reference, describes a non-interactive magnetic brush development method employing permanently magnetized carrier beads operating with a rotating multipole magnet within a conductive and nonmagnetic sleeve. Magnetic field lines form arches in the space above the sleeve surface and form chains of carrier beads. The developer chains are held in contact with the sleeve and out of direct contact with a photoreceptor by gradients provided by the multipole magnet. As the magnet rotates in one direction relative to the sleeve, the magnetic field lines beyond the sleeve surface rotate in the opposite sense, moving carrier bead chains in a tumbling action which transports developer material along the sleeve surface. The strong mechanical agitation effectively dislodges toner particles generating a rich powder cloud which can be developed to the adjacent photoreceptor surface under the influence of development fields between the sleeve and the electrostatic image. However, such radial flow of developer material occurs over the full surface of the sleeve; hence, a stripping device such as a skiving blade must be included for removing spent developer before it returns to the development zone.
In conceiving the present invention, I have found that the conventional approach to construction and operation of a development system, and especially the above-described development system, suffer from the following undesirable phenomena.
Firstly, the magnetic brush development configuration disclosed in Kaukeinen et al. does not lend itself readlly to the construction and operation of a development system located laterally (at what is known as a xe2x80x9cthree o""clockxe2x80x9d or a xe2x80x9cnine o""clockxe2x80x9d position) with respect to a vertically-oriented photoreceptor surface.
Secondly, as illustrated in FIG. 1, it may be observed that the provision of agitated bead chains according to at least one of the foregoing approaches will result in linear ridges, or xe2x80x9cpilesxe2x80x9d, of developer material distributed about a developer sleeve 100 due to the high density of bead chains approximately at the midpoint of each magnet 120. (Corresponding valleys, or xe2x80x9ctroughsxe2x80x9d, of developer material thus locate at the intersections of adjacent magnets 120.) As a consequence of this effect, the resulting developer chains accumulate over the developer roll in linear piles having varying peaks and troughs that are arranged in parallel to the central longitudinal axis of the magnetic developer roll.
In typical practice, the magnetic developer roll is operated with associated devices, such as a skiving blade, that engage the developer sleeve 100 in order to remove unused developer and toner material from the developer sleeve 100. Similar devices may be located adjacent the developer sleeve to meter fresh developer material onto the developer sleeve 100 so as to effect replenishment of fresh developer material. In typical practice, such blades are oriented in parallel with the same central longitudinal axis of the magnetic developer roll.
In conceiving the present invention, I have found that the above-described development systems suffer from the following undesirable phenomena.
The magnetic brush height formed by the developer mass in the magnetic fields on the sleeve surface in the aforementioned types of development systems is periodic in thickness and is statistically noisy as a result of complex carrier bead agglomeration and filament exchange mechanisms that occur during operation. Accordingly, substantial clearance must be provided in the development gap to avoid photoreceptor interactions through direct physical contact. The use of a closely spaced developer layer, which is critical to high fidelity image development, is precluded.
The magnetic pole spacing cannot be reduced to an arbitrarily small size because allowance for the thickness of the sleeve and a reasonable mechanical clearance between the sleeve and the rotating magnetic core sets a minimum working range for the magnetic multipole forces required to both hold and tumble the layer of developer material on the sleeve. Since the internal pole geometry defining the spatial wavelength of the tumbling component also governs the magnitude of the holding forces for the developer material at any given range, there is limited design freedom available to satisfy the opposing system requirements of short spatial wavelength and strong holding force.
Relative rotation of the magnetic developer roll and the developer sleeve will rotate successive ones of the magnets within the developer sleeve and thus under the engaging edge of the skiving blade. A corresponding movement of successive linear piles of developer material move along the exterior of the developer sleeve. As a result, the skiving blade will periodically be impacted by the entire length of a linear pile of developer material, whereupon the development system undergoes substantial increase in mechanical stress, which is periodic due to the rapid succession of developer pile masses encountered by the skiving blade. During each stress peak, the skiving blade, magnetic developer roll, developer material, along with the motor drive and any respective mechanisms including motor drive bearings, will experience a significant increase in mechanical force. The motor drive also undergoes a significant peak demand for drive torque. Additional undesirable effects include a likelihood of vibration in the system, an increase in imaging artifacts due to motion disturbances, degradation of the carrier and toner (presumably due to the pile/blade impact), and a propensity for temporal and spatial variations in the efficiency of the skiving blade. Such a skiving blade can become contaminated and thus falter, and developer additives are required to keep it clean. Such skiving action can also abrade both the skiving blade and the developer sleeve. This magnetic developer roll can also become impractical when constructing a wide body configuration (e.g., a roll length that exceeds approximately 17 inches), as the aforementioned mechanical stress can be expected to increase dramatically as the magnetic developer roll increases in length.
The present invention obviates the problems noted above by providing a development system that employs a magnetic developer roll featuring a magnetic core operable for relative rotation within a developer sleeve. The magnetic core includes a plurality of magnetic core pole segments that are tightly arranged on the magnetic core in an alternating pole pattern. The development system is constructed such that orientation of the magnetic developer roll to a vertically oriented photosensitive medium such as a photoconductor belt or drum may be laterally positioned with respect to the vertically oriented photosensitive medium. Accordingly, and in a principal feature of the invention, the magnetic field gradient of the magnetic core is blocked by use of magnetic shunts located about the magnetic core such that certain corresponding portions of the developer material layer most proximate to the magnetic shunts are released from the developer sleeve and fall under the influence of gravity into a developer material sump for recirculation in the developer material sump.
The contemplated development system is thus optimized for provision of a development zone locatable at a three o""clock or nine o""clock position with respect to a photoconductor belt. The development system includes a developer material sump, a magnetic developer roll having a rotatable sleeve enclosing a magnetic core, and a developer material delivery system including a mixing assembly and a developer material metering device for providing a metered supply of developer material from the developer material sump to the sleeve. The developer material is moved from the mixing assembly and into the metering device and for aiding transport of the developer material to the sleeve. In a preferred embodiment, developer material is provided to the sleeve from a metering slot and the developer material dispensed from the slot is attracted to the sleeve due to the magnetic field provided by the magnetic core.
A slot located within the metering device and proximate to the loading zone thereby forms a thin, pre-defined layer of developer material on the sleeve exterior surface. The magnetic core in the rotating developer roll generates a magnetic field to move the layer of developer material along the sleeve exterior surface to the development zone, where a portion of the layer is agitated, that is, the portion of the layer of developer material proximate to the magnetic core and thereby subject to the magnetic field gradient of the magnetic core. The toner particles are dislodged from the carrier beads to aid in the formation of a toner cloud in the development zone. Continued rotation of the sleeve brings the layer of developer material to a drop-off zone adjacent the shunts where the magnetic field has declined sufficiently to allow the developer material to fall from the sleeve into the sump.
Accordingly, the magnetic developer roll may be optimized for a particular developer layer height and agitation, for more efficient development of a latent image. Additional benefits include: extended operating latitudes, improved development efficiency, and improved mixing of developer material.
Furthermore, a developer material stripping device is no longer necessary, because spent developer material falls from the sleeve at the drop-off zone. The contemplated development system thereby experiences a smoother rotational operation, and a significant reduction in the mechanical forces and stresses described above, along with a concomitant reduction in vibration, imaging artifacts, carrier and toner degradation, and other deficiencies in the performance of the system.
Thus, a development system constructed according to this invention is not only compact and robust but also permits use of less costly components, such as a compact magnetic core, and a motor drive of lower drive torque.
An apparatus for non-interactive development of electrostatic images may be constructed to include an image bearing member bearing an electrostatic image; two-component developer material comprising toner and hard magnetic (permanently-magnetized) carrier beads, a magnetic developer roll, a rotatable magnetic core having multiple pole segments, and a cylindrical developer transporting sleeve enclosing and rotating about the rotatable magnetic core.
In a particular feature of the present invention, the sleeve exhibits a uniform, predefined sleeve radius and the magnetic core is located within the sleeve with at least a first magnetic shunt being located between the magnetic core and the sleeve, and a second magnetic shunt being located between the exterior of the sleeve and proximate portions of the mixing assembly. Accordingly, the magnetic core is optimally located for agitating a portion of the developer layer of the two component developer material, the agitated portion being spaced close to but not contacting with the image bearing member. Thus, the developer layer is transported about the sleeve from a loading zone, to and within the development zone, where it is advantageously subject to agitation induced by the field gradients provided by the magnetic core, and therefrom to a drop-off zone.
In another feature of the present invention, the layer of developer material will progress from the loading zone through the development zone to a drop-off zone. This feature is useful in constructing a development system according to the present invention for a xe2x80x9cwide-bodyxe2x80x9d reprographic system design which have photosensitive members exhibiting as much as 36 inches in width. A development system for such a wide body reprographic system can require, for example, an input of developer material into the development system housing at one side with a subsequent output of developer material at the opposing side of the development system housing.
In another feature of the invention, a development system for developing an image with developer material may be constructed in a more compact unit.
In another feature of the invention, an imaging system for providing image-on-image, non-interactive development of electrostatic images may be constructed to include an image bearing member bearing an electrostatic image; a housing containing developer material, a magnetic developer roll for transporting the developer material from the housing to the image.
Embodiments of the present invention are both robust and permit a spacing between a development system and the electrostatic image that is small enough to eliminate or significantly reduce image defects associated with fine lines and edges.